Almkvist's unimodality conjecture for two-variable Hilbert functions

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Let m,nm,n be positive integers, let R=Q[x1,…,xn]R={\mathbb Q}[x_1,\ldots,x_n] with its standard grading, and write ei=ei(x1,…,xn)e_i=e_i(x_1,\ldots,x_n) for the elementary symmetric functions and ei(m)=ei(x1m,…,xnm)e_i(m)=e_i(x_1^m,\ldots,x_n^m). Define

A(m,n)=Q[e1,…,en](e1(m),…,en(m))A(m,n)=\frac{{\mathbb Q}[e_1,\ldots,e_n]}{(e_1(m),\ldots,e_n(m))}

and let H(m,n)=(H(m,n)k)k=0dH(m,n)=(H(m,n)_k)_{k=0}^d be the coefficient sequence of its Hilbert polynomial.

Almkvist's conjecture. For each mm, the Hilbert function H(m,n)H(m,n) is unimodal for all n≥11n\geq 11. Moreover, if mm is even, then H(m,n)H(m,n) is unimodal for all nn.

These Hilbert functions also enumerate a two-parameter family of integer partitions and are related to the unimodality of graded Artinian complete intersections. The conjecture is attributed to G. Almkvist and is presented here without a stated resolution.

References

Primary source

Nancy Abdallah and Chris McDaniel, “Lattice Paths, Lefschetz Properties, and Almkvist's Conjecture in Two Variables”, arXiv:2404.05098 (2024).

Additional references

4 papers in this index state this conjecture (2018–2024). The statement above is taken from the most recent of them; the others are arXiv:2311.03081, arXiv:2304.01032, arXiv:1807.05869.

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